materials Article Virtual Experiments of Particle Mixing Process with the SPH-DEM Model Siyu Zhu, Chunlin Wu and Huiming Yin * Department of Civil Engineering and Engineering Mechanics, Columbia University, 610 S.W. Mudd, 500 West 120th Street, New York, NY 10027, USA; [email protected] (S.Z.); [email protected] (C.W.) * Correspondence: [email protected]; Tel.: +1-212-851-1648 Abstract: Particle mixing process is critical for the design and quality control of concrete and composite production. This paper develops an algorithm to simulate the high-shear mixing process of a granular flow containing a high proportion of solid particles mixed in a liquid. DEM is employed to simulate solid particle interactions; whereas SPH is implemented to simulate the liquid particles. The two-way coupling force between SPH and DEM particles is used to evaluate the solid-liquid interaction of a multi-phase flow. Using Darcy’s Law, this paper evaluates the coupling force as a function of local mixture porosity. After the model is verified by two benchmark case studies, i.e., a solid particle moving in a liquid and fluid flowing through a porous medium, this method is applied to a high shear mixing problem of two types of solid particles mixed in a viscous liquid by a four-bladed mixer. A homogeneity metric is introduced to characterize the mixing quality of the particulate mixture. The virtual experiments with the present algorithm show that adding more liquid or increasing liquid viscosity slows down the mixing process for a high solid load mix. Although the solid particles can be mixed well eventually, the liquid distribution is not homogeneous, especially when the viscosity of liquid is low. The present SPH-DEM model is versatile and suitable for virtual experiments of particle mixing process with different blades, solid particle densities and sizes, and liquid binders, and thus can expedite the design and development of concrete materials Citation: Zhu, S.; Wu, C.; Yin, H and particulate composites. Virtual Experiments of Particle Mixing Process with the SPH-DEM Keywords: smoothed particle hydrodynamics; discrete element method; Darcy’s law; homogeneity Model. Materials 2021, 14, 2199. metric; particle mixing process https://doi.org/10.3390/ma14092199 Academic Editor: Bora Gencturk 1. Introduction Received: 12 March 2021 Mixing of granular materials is crucial in a broad range of industrial processes, in- Accepted: 20 April 2021 Published: 25 April 2021 cluding constructional material production, advanced composite manufacture, mineral processing, plastics manufacturing, ceramic component, pharmaceutical tablets, and food Publisher’s Note: MDPI stays neutral products. Mix uniformity plays a critical role for the overall performance. Many exper- with regard to jurisdictional claims in imental approaches have been employed in quality control of the mixing process, such published maps and institutional affil- as sampling, visual tracking, magnetic resonance imaging, rheometer measurements, etc. iations. Among others, Khan et al. [1] used high speed image analysis to track the tracer particles, through which the dispersion coefficients are obtained according to fluctuating velocity components and partial mean-free path. Although these methods provide valuable insight of the mixing processes, their applications are limited due to the low consistency and high labor costs [2–4]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Moreover, it is time-consuming to develop a new particulate composite through such This article is an open access article a trial-error method. Therefore, numerical simulation becomes an alternative to investigate distributed under the terms and granular flow dynamics.For solid applications, Orefice and Khinast applied DEM to study conditions of the Creative Commons the transporting process of screw conveyors with different inclinations and filling levels [5]. Attribution (CC BY) license (https:// And Sinaie simulated the size effects of concrete samples with DEM [6]. He et al. [7] creativecommons.org/licenses/by/ investigated the interesting phenomenon of axial segregation and its key factors of the 4.0/). mixing process in a rotating drums. In [8], the authors extended the original DEM to an Materials 2021, 14, 2199. https://doi.org/10.3390/ma14092199 https://www.mdpi.com/journal/materials Materials 2021, 14, 2199 2 of 24 explicit model for identical superellipses that 2D discrete Fourier is applied to approximate the overlapping part explicitly. Among mixing problems, liquid-solid mixing becomes more common, and a reliable model to simulate the solid-liquid mixing process is highly demanded. Different approaches have been proposed to tackle the liquid-solid granular flow problems, which could be classified mainly to three categories: continuous media models, discrete particles models, and their combination [2,4,9]. Continuous media models treat both solid and liquid phases as interpenetrating continua. This scheme works well for problems with small deformation, but often fails in problems with large deformation, such as mixing problems. In comparison, discrete particle methods can simulate the dynamics of particulate mixtures and are capable of dealing with moving boundary problems in the mixing process, such as the Molecular Dynamic (MD), Discrete Element Method (DEM), Smooth Particle Hydrodynamics (SPH), and Dissipative Particle Dynamics (DPD). In these methods, some specific treatments are generally required to evaluate the coupling between solid particles and liquid particles [10–12]. Other methods combine continuous media and discrete particles, such as CFD cou- pling DEM [13,14], where CFD is implemented to simulate fluid as a continuum medium and DEM is applied to simulate solid particles as meshless discrete particles. For mix- ing problems where the solid load is high, but not high enough to apply liquid bridge models [15–17], the coupled CFD-DEM approach can be time consuming in modeling the moving boundary and discretizing the complex geometry, and SPH provides a practical method to simulate the liquid phase among a large number of solid particles with only a small number of liquid particles. To tackle mixing problems with an intermediate to high solid load, SPH-DEM coupling is a promising approach. Cleary [12] proposed a one-way coupling SPH-DEM model to simulate slurry transport. Sun [18] introduced pressure difference and drag forces between liquid-solid interactions, and validated them with rotating drums filled with solid-liquid mixtures. Jonsen [19] employed the DEM stiffness to couple SPH with DEM, and obtained expected torque in a rotation drum; while Canelas [20] applied the SPH momentum equation to the interactions between solid and liquid particles and demonstrated the feasibility by comparing the simulation with the experiments in free surface solid-fluid flows. Robinson [21] introduced the local average N-S equation into coupling methods and validated the scheme with solid sedimentation in liquid. Robin’s scheme was later improved and applied to a high-shear mixing problem by Kwon [22] and Qi [23]. In comparison to DEM particles added to SPH liquid, some preliminary work has been conducted towards SPH liquid flow in porous media. Shao [24] and Zhu [25] treated porous structures as boundaries for SPH particles, which are in a smaller size compared to the porous structures. Jiang [26] proposed L-J potential between SPH and solid skeleton particles and demonstrated that the scheme can reproduce Kozeny’s formula of perme- ability. Peng [27] presented resistance interactions between solid and liquid SPH particles based on Darcy’s law and validated the method with a porous media case study. However, it is still challenging for the coupled SPH-DEM method to simulate particle mixing with a high solid load due to the difficulty of calculating the interaction force between SPH and DEM particles, because most previous models were built under the assumption that solid particles are fully saturated by the liquid phase. This work aims at the solid-liquid mixing problems with very high solid loads, say up to 90% in volume. Here the key issue is how to calculate the interaction between liquid and solid particles. To compute the local liquid density, solid particles are considered to be rigid without any overlap with each other. Rather than excluding solid particles by dividing porosity in the density computation [21], this paper includes solid particles in density computation to avoid potential instability at a high solid load. SPH momentum equation is employed for the interaction between solid and liquid particles, in which the effective viscosity coefficient varies according to its local porosity following Darcy’s Law without using any artificial drag forces. This approach considers the nature of particle interactions in mesoscale, and is more efficient and effective for computation. Materials 2021, 14, 2199 3 of 24 In the following, the numerical algorithm and formulation of the two-way DEM-SPH coupling method are introduced first. Two benchmark studies, i.e., (1) a solid particle sedimentation among liquid and (2) liquid flow through a porous media, are presented to verify the model for the parameter calibration. Subsequently, the present method is applied to high shear particle mixing problems, and a series of parametric studies are conducted. The local average mixing index is the main criterion to evaluate the homogeneity of the mixture, which will be used to optimize the particle mixing process